The present invention relates to a method for measuring the volumetric water content of mineral and/or organic mixtures by means of dielectric measurements with the aid of a probe including at least two measuring electrodes. The invention further relates to an apparatus for measuring the volumetric water content of mineral and/or organic mixtures by means of dielectric measurements with the aid of a probe having a probe body carrying at least one electrode arrangement in the form of a two-conductor or three-conductor system.
Meteorological models require a precise knowledge of the water contents in the ground soil since, due to the great heat retention capacity of water compared to air, the exchange of energy between the ground and the atmosphere is transacted primarily by evaporation and condensation of water. In hydrology, a precise knowledge of the water content of the ground forms the basis of all planning. In agriculture, optimum watering is impossible without a precise determination of water content of the soil. Insufficient watering leads to damage of the plants while too extensive watering results in a leaching out of nutrients. In drying processes, a precise knowledge of water content produces advantages of economy since drying processes are very energy intensive.
Generally, determinations of moisture content of soil and other mineral and/or organic mixtures are made using probes to determine their dielectric constant. In DE 3,402,708.A1 and in the operating instructions for the BF 610 instrument produced by Philipp Schenk GmbH Wien & Co. KG, Jedleseer Strasse 59, A-1210 Vienna, Austria, two devices are described for measuring soil moisture. Each of these devices is pushed into the ground in order to make the measurements. Pushing such a device into the soil changes the density of the soil. Because the compression is effected concomitantly with a reduction in the percentage of air in the soil, the water which is present becomes concentrated in a smaller volume. That causes the measurement to be incorrect. Moreover, the change in density remains in effect for a very long time, and the measurement is no longer representative of an uninterferred-with volume.
From the measured values of dielectric constant, the moisture content of the soil can be determined only by way of a calibration curve which can only be prepared in the laboratory using a complicated procedure. Moreover, the determination must be made separately for each type of soil. For this purpose, a representative soil sample of a particular type is drawn and dried completely. After adding different defined amounts of water to the sample, measurements on the sample are made and the results used to form the calibration curve.
Ground soil is an important example of mixtures containing three or more components, to which the invention is applicable. A fraction of the volume of a soil mixture to be measured is the dry soil B; in addition, a fraction W is water and a fraction L is air so that: EQU L+W+B=1.
The dielectric constant of this mixture, .epsilon..sub.g1, is equal to the weighted sum of the dielectric constants of the components of the mixture. Thus, EQU .epsilon..sub.g1 =B.multidot..epsilon..sub.B +W.multidot..epsilon..sub.W +L.multidot..epsilon..sub.L
wherein .epsilon..sub.B is the dielectric constant of the dry soil, .epsilon..sub.W is the dielectric constant of water, and .epsilon..sub.L is the dielectric constant of air. Generally, B, .epsilon..sub.B and W of this equation are unknown. An error in the results obtained with the prior art methods resides in the fact that B.multidot..epsilon..sub.B and L.multidot..epsilon..sub.L are neglected. The smaller the water content, the greater is this error. Many probe manufacturers therefore recommend calibration of the probe for the respective mixtures according to the water content. Such a calibration requires that several very precise samples be taken. Moreover, the measurements involved are difficult to carry out accurately since, as noted above, they are subject to error if the samples are compressed as they are taken. Further, the measurements require the use of time consuming and energy intensive heating processes.
If the area of the upper surface of the probe (the probe cover) is approximately the same as the area of a vertical projection of the volume of the soil mixture to be measured, rain water flowing off from the cover falsifies the measuring result. The concentration of rain water in the vicinity of the probe also causes difficulty in performing the measurements because the outflowing water washes out channels between the soil and the probe wall.